Method of manufacturing torsion bar for vehicle steering device and torsion bar

ABSTRACT

A method of manufacturing a torsion bar includes a first shaping step of reducing a diameter of a round bar steel material according to cold drawing work so as to have a hardness including an inside through a surface layer of 320 or more to 450 or less by Vickers hardness (HV), and a second shaping step of cutting a midway area of the steel material in the longitudinal direction so as to obtain a spring portion.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a torsion barfor a steering apparatus of a vehicle such as an automobile and thetorsion bar.

BACKGROUND ART

As disclosed in specifications of U.S. Pat. No. 4,699,174 and U.S. Pat.No. 4,550,597, steering apparatuses such as power steerings provided tovehicles such as automobiles assist rotating operation power using apower source such as a motor or a hydraulic mechanism at the time of therotating operation of a steering wheel. A torsion bar of the presentinvention is unitized in order to control the operation of the powersource.

The torsion bar is used on a path for transmitting a rotating power ofthe steering wheel to a steering gear box. When the steering wheel isrotated, the torsion bar is twisted, so that a difference is generatedbetween a rotation phase on the side of the steering wheel and arotation phase on the side of the steering gear box. The phasedifference is suitably detected so that the operation of the powersource is controlled.

In the torsion bar for the steering apparatus, both ends in alongitudinal direction are joint portions which are supported by anobject to be attached, and a midway portion in the longitudinaldirection is a spring portion which allows torsion. Such a kind oftorsion bars include a torsion bar in which its entire shape is a roundbar, for example, joint portions on both ends in its longitudinaldirection have large diameter, and a spring portion on a midway portionin the longitudinal direction has a small diameter.

The torsion bar used for the above application requires predeterminedfatigue strength against repeated torsion. The fatigue strength againstrepeated torsion is determined so as to satisfy a condition such thatwhen a torsion stress of 300 MPa or more is applied, torsion can becarried out at least 500000 times.

A method of manufacturing the prior torsion bar is explained. A roundbar steel material with low hardness first undergoes a cutting work soas to have an outer shape which is close to a finished product. Thesteel material having the outer shape close to the finished product bythe cutting undergoes high-temperature heat treatment so as to havepredetermined high hardness. Finally, warping of a matrix due to theheat treatment is corrected and distortion is removed, so that an outerdiameter is adjusted. In such a manner, the prior torsion bar is firstshaped into the outer shape close to the finished product of the torsionbar, and then its hardness is adjusted so as to be high.

A main object of the present invention is to provide a method ofmanufacturing a torsion bar for first giving treatment which heighten ahardness of an entire round bar steel material according to cold drawingwork, and working the round bar steel material into an outer shape closeto a finished product of a torsion bar, so as not to requirehigh-temperature heat treatment for heightening the hardness.

DISCLOSURE OF THE INVENTION

The present invention is a method of manufacturing a torsion bar havinga spring portion in a midway area in a longitudinal direction and jointportions on both ends in the longitudinal direction, including: a firstshaping step of reducing a diameter of a steel material by reduction ofarea within a predetermined range according to cold drawing work so asto heighten a hardness of the entire steel material within apredetermined range; and a second shaping step of cutting the midwayarea of the steel material in the longitudinal direction so as to obtainthe spring portion. The cold drawing is for plastically working aworkpiece at a low temperature not higher than an annealing temperature,and reducing a diameter of a steel material via a die having a graduallysmaller diameter. Cutting is one of removing work for removing anunnecessary portion of a workpiece as using a machine tool and a tool soas to work the workpiece into desired shape and dimension. The cuttingincludes turning and grinding.

As one preferable mode of the present invention, the hardness of theentire steel material after the cold drawing work is 320 or more to 450or less by Vickers hardness (HV).

As one more preferable mode of the present invention, the reduction ofarea is 12 to 15%.

In the present invention, high-temperature heat curing treatment is notcarried out unlike prior examples, but plastic deformation for reducinga diameter of a steel material using the cold drawing work is executedso that the entire hardness is adjusted to be large. That is to say,since only a dimension for the plastic deformation of the steel materialis determined and the heat treatment, which costs the most in the priorexamples, is not carried out. The present invention is, therefore,advantageous to a reduction in manufacturing cost.

As disclosed in Japanese Patent Application Laid-Open No. 3-189043(1991), a torsion bar here is obtained in such a manner that a steelmaterial whose diameter is larger than a finished diameter of jointportions of the torsion bar is used, and only a spring portion of thetorsion bar is swaged. It is, however, difficult to control a length ofthe spring portion accurately according to the swaging. Further, sinceafter-treatment for the swaging is not carried out, accuracy ofcircularity and outer diameter of the spring portion is low.

As one more preferable mode of the present invention, the blueing stepmay be given to the steel material before or after the second shapingstep. In this case, the hardness of the steel material can be heightenedcomparatively simply. Further, as the steelmaterial, one of SUP12 andSWRH82B of JIS can be selected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view illustrating a torsion bar according to a bestmode of the present invention.

FIG. 2 is a step diagram illustrating a procedure for manufacturing thetorsion bar in FIG. 1.

FIG. 3 is a sectional view illustrating a hardness measuring position ofa matrix.

FIG. 4 is a graph showing a result of inspecting the hardness of thematrix after a first shaping step.

FIG. 5 is a graph showing a result of inspecting the hardness of thematrix after a second shaping step.

FIG. 6 is a step diagram illustrating a procedure for manufacturing thetorsion bar according to another mode of the present invention.

FIG. 7 is a graph showing a result of inspecting the hardness of thematrix after the blueing step.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1 to 5, a method of manufacturing a torsion baraccording to the best mode of the present invention is explained. Atorsion bar 1 has a round bar shape from an overall view. The torsionbar 1 has such a shape that a midway area in a longitudinal directionhas a spring portion 2 with small diameter, and both end areas in thelongitudinal direction have joint portions 3 and 4 with a largediameter. The spring portion 2 has a circular section and has a uniformdiameter in the longitudinal direction. A diameter of the joint portions3 and 4 is larger than the diameter of the spring portion 2, and theyhave a circular section and a uniform diameter in the longitudinaldirection. The size of the diameter of the spring portion 2, the jointportions 3 and 4 is not limited to this mode. A length relationshipbetween the spring portion 2 and the joint portions 3 and 4 may besuitably determined according to automobiles which use the torsion bar.The section of the joint portions 3 and 4 is not necessarily circular.

Portions connecting the spring portion 2 and the joint portions 3 and 4have a round curved surface so that the diameter is gradually widened toa direction from the spring portion 2 to the joint portions 3 and 4.When a steering wheel is rotated in a steering apparatus such as a powersteering, the spring portion 2 is twisted so as to generate a differencebetween a rotation phase on the side of the steering wheel and arotation phase on the side of a steering gear box. The spring portion 2allows the torsion. The joint portions 3 and 4 are jointed to an objectto be attached in the steering apparatus. For attaching to such anobject to be attached, the joint portions 3 and 4 are provided with aspline, a serration, a through hole piercing in a radial direction, orthe like (not shown), if necessary.

With reference to FIG. 2, the method of manufacturing the torsion bar 1is explained. This manufacturing method includes a first shaping step 11and a second shaping step 12. In this mode, for example, one of springsteels SUP12 of JIS and a high carbon steel wire rods SWRH82B of JIS isselected as a matrix of the torsion bar 1 in which Vickers hardness (HV)is less than 320. A round bar steel material, whose diameter is set tobe larger than a finished diameter of the joint portions 3 and 4 ifnecessary, is prepared as the matrix. Hardness of SUP12 and SWRH82B ofJIS is about 200 by Vickers hardness (HV).

At the first shaping step 11, the prepared round bar steel materialundergoes cold drawing work, so that its entire diameter in thelongitudinal direction is reduced to be uniform.

At the first shaping step 11, reduction of area for the round bar steelmaterial as the matrix is managed, so that predetermined hardness issecured. When the cold drawing work is carried out at a plurality oftimes, however, the reduction of area at each time and total reductionof area should be set suitably.

The reduction of area γ(%) is represented by a following equation aspublicly known.γ={(A ₀ −A _(n))/A ₀}×100

In the above equation, A₀ denotes a cross section of the matrix beforethe work, and A_(n) denotes a cross section of the matrix after thefinal work. Incidentally, when SUP12 and SWRH82B of JIS are used, thereduction of area is set to 12 to 15%, so that the hardness of an areaof the matrix including an inside through a surface layer is set to 320or more by Vickers hardness (HV).

At the second shaping step 12, the midway area of the round bar steelmaterial with reduced diameter in the longitudinal direction is cut soas to have a smaller diameter than that of the both end areas in thelongitudinal direction, thereby shaping the round bar steel materialinto the outer shape close to a finished product having the springportion 2 and the joint portions 3 and 4.

In this case, at the first shaping step 11, the diameter of the roundbar steel material with reduced diameter shaped by cold drawing work isset to a dimension slightly larger than a finished diameter of the jointportions 3 and 4. At the second shaping step 12, not only the springportion 2 but also the joint portions 3 and 4 are cut, so that thediameter of the joint portions 3 and 4 is managed.

The hardness on some places of the matrix in a depthwise direction isinspected after the first shaping step 11 and after the second shapingstep 12 in a following manner.

The hardness of the joint portions 3 and 4 is measured, and measuringpositions are, as shown in FIG. 3, five positions P1 to P5 in the depthwise direction. The position P1 is separated by 0.5 mm from a positionP0 (surface), the position P2 is separated by 1.0 mm from the positionP0, the position P3 is separated by 2.0 mm from the position P0, theposition P4 is separated by 3.0 mm from the position P0, and theposition P5 is separated by 4.0 mm from the position P0.

Examples 1 to 4 are prepared as samples. The examples 1 and 2 use SUP12of JIS as a matrix. The examples 3 and 4 use SWRH82B of JIS as a matrix.Reduction of area in the examples is set to 12.6%.

The hardness of the examples at the time of completion of the firstshaping step 11 is shown in FIG. 4 and Table 1. As mentioned above,places where the hardness is measured are the joint portions 3 and 4,but at the first shaping step 11, since an outer diameter of the entirelongitudinal direction of the round bar steel material as the matrix isreduced into an approximately uniform dimension, it is considered thatthe hardness of the joint portions 3 and 4 is approximately same as thehardness of the spring portion 2.

In the examples 1 to 4, accordingly, the hardness of the entire portionincluding the inside through the surface of the round bar steel materialis 320 or more to 450 or less by Vickers hardness (HV). PreferableVickers hardness (HV) of the entire round bar steel material is 350 ormore to 430 or less. TABLE 1 Measuring position from outer diameterExample 1 Example 2 Example 3 Example 4 Unit (mm) Unit (HV) P1→0.5 340360 390 380 P2→1.0 370 380 400 390 P3→2.0 370 370 390 400 P4→3.0 340 350380 370 P5→4.0 360 340 350 325

The hardness at the time of completion of the second shaping step 12 isshown in FIG. 5 and Table 2.

In the examples 1 to 4, accordingly, the hardness of the round bar steelmaterial including the inside through the surface is 320 or more to 450or less by Vickers hardness (HV). TABLE 2 Measuring position from outerdiameter Example 1 Example 2 Example 3 Example 4 Unit (mm) Unit (HV)P1→0.5 350 360 380 370 P2→1.0 370 360 370 380 P3→2.0 370 370 390 360P4→3.0 350 350 350 360 P5→4.0 340 360 330 350

In the case where the SUP12 and SWRH82B of JIS are used as the matrix,the reduction of area at the first shaping step 11 is set to 12 to 15%,preferably to 12.6%, so that the hardness of the spring portion 2 andthe joint portions 3 and 4 are set to 320 or more by Vickers hardness(HV).

When the above-mentioned hardness is secured, tensile strength of thetorsion bar 1 is set to 100 to 165 kg/mm², and fatigue limit underrotary bending is set to 48 to 70 kg/mm². Fatigue strength againstrepeated torsion can satisfy a condition such that when a torsion stressof 300 MPa or more is applied, the repeated torsion can be carried outat least 5×10⁶ times similarly to the prior matrices. Incidentally, whenthe above fatigue strength against repeated torsion is secured, thetorsion bar 1 fulfills the function in the steering apparatus such as apower steering mounted to a vehicle such as an automobile.

In the case of the torsion bar 1 of the above-mentioned mode, the matrixof the torsion bar 1 is selected and is plastically deformed withoutundergoing heat curing treatment. As a result, cost of the matrix mayrise higher than the prior matrices, but since the heat curing treatmentis not carried out, production efficiency can be improved andmanufacturing cost can be reduced. In the manufacturing method of thepresent invention, therefore, the torsion bar 1 with high quality whichsatisfies a predetermined performance can be manufactured at acomparatively low price.

The present invention is not limited only to the above mode, and variousapplications and modifications are considered.

(1) In the above mode, after the second shaping step 12, blueing workwhich is a well known technique may be executed. That is to say, asshown in FIG. 6, the first shaping step 11, the second shaping step 12and the blueing step 13 may be executed in this order as themanufacturing steps of the torsion bar 1.

The first and second shaping steps 11 and 12 are the same as those inthe aforementioned mode, the explanation thereof is omitted. At theblueing step 13, the matrix is heated at ambient temperature of 200 to350° C., for example. In this case, since it is an object of the blueingstep to remove a residual stress in cold-formed products, the treatmenttemperature is 200 to 350° C. as mentioned above, namely, comparativelylow. For this reason, this step is different from the high-temperatureheat treatment for heightening hardness of a prior torsion bar shapedinto a finished product.

In this mode, at the time of completion of the blueing step 13, as shownin FIG. 7 and following Table 3, the hardness of the spring portion 2and the joint portion 3 and 4 can be heightened to 360 or more bybickers hardness (HV). TABLE 3 Measuring position from outer diameterExample 1 Example 2 Example 3 Example 4 Unit (mm) Unit (HV) P1→0.5 370390 390 390 P2→1.0 380 390 390 390 P3→2.0 380 390 390 390 P4→3.0 360 370370 380 P5→4.0 370 370 360 360

(2) In the mode explained in (1), the blueing step 13 may be executedbefore the second shaping step 12.

In the above modes, after the final step, the round bar steel materialwhich is shaped into the outer shape close to the finished product iscut, so that quality such as accuracy of an outside dimension can befurther improved. In the manufacturing method explained in (1), thecutting may be carried out between the second shaping step 12 and theblueing step 13. Since the cutting is not for removing warpage anddistortion due to the heat curing process unlike the prior example, itonly takes a short time.

As is clear from the above modes, in the manufacturing method of thepresent invention, the steel material, which has a shape close to thefinished product of the torsion bar, does not undergo the heat treatmentfor heightening the hardness which causes high cost unlike the priorexample. In the manufacturing method of the present invention,therefore, the production efficiency can be improved and themanufacturing cost can be reduced. In the manufacturing method of thepresent invention, therefore, the torsion bar which satisfies thepredetermined performance can be manufactured at comparatively low cost.

INDUSTRIAL APPLICABILITY

The present invention can be applied to the torsion bar to be used inthe steering apparatus such as a power steering mounted to a vehiclesuch as an automobile.

1. A method of manufacturing a torsion bar having a spring portion in amidway area in a longitudinal direction and joint portions on both endsin the longitudinal direction, comprising: a first shaping step ofreducing a diameter of a steel material by reduction of area within apredetermined range according to cold drawing work so as to heighten ahardness of the entire steel material within a predetermined range; anda second shaping step of cutting the midway area of the steel materialin the longitudinal direction so as to obtain the spring portion.
 2. Themethod of manufacturing the torsion bar according to claim 1, whereinthe hardness of the entire steel material after the cold drawing work is320 or more to 450 or less by Vickers hardness (HV).
 3. The method ofmanufacturing the torsion bar according to claim 1, wherein thereduction of area is 12 to 15%.
 4. The method of manufacturing thetorsion bar according to claim 1, wherein at the first shaping step, thediameter of the shaped steel material with the diameter being reduced bythe cold drawing work is set so as to be slightly larger than a finisheddiameter of the joint portions, and at the second shaping step, not onlythe spring portion but also the joint portions are cut so that thediameter of the joint portions is managed.
 5. The method ofmanufacturing the torsion bar according to claim 1, wherein at the firstshaping step, the cold drawing work is carried out at a plurality oftimes, and in an equation γ={(A0−A1)/A0}×100 in which the reduction ofarea of the steel material at each time is γ(%), a cross section of thesteel material before the drawing work is A0, and a cross section of thesteel material after final work of the drawing work is A1, the reductionof area is set to 12 to 15%, and the Vickers hardness (HV) is set to 320or more.
 6. The method of manufacturing the torsion bar according toclaim 1, further comprising the blueing step of giving blueing work tothe steel material before or after the second shaping step.
 7. A torsionbar having a spring portion in a midway portion in a longitudinaldirection and joint portions on both ends in the longitudinal direction,said torsion bar being manufactured by a first shaping step of reducinga diameter of a steel material by reduction of area within apredetermined range according to cold drawing work so as to heighten ahardness of the entire steel material within a predetermined range andthe second shaping step of cutting the midway area of the steel materialin the longitudinal direction so as to obtain the spring portion.
 8. Thetorsion bar according to claim 7, wherein the hardness of the entiresteel material after the cold drawing work is 320 or more to 450 or lessby Vickers hardness (HV).
 9. The torsion bar according to claim 7,wherein the reduction of area is 12 to 15%.